LiMPO4 (M represents transition metals), which is a olivine-type lithium metal phosphate and has a larger electric capacity, may be used as a cathode active material for lithium secondary batteries which is not as expensive as that using LiCoO2 which has conventionally been widely used as a cathode active material for lithium secondary batteries in the related art. Accordingly, LiMPO4 has been expected as a cathode active material of a lithium secondary battery and particularly, as a cathode active material of a large-scaled lithium secondary battery for vehicles. (Patent Documents 1 and 2)
Various synthesis methods, such as a solid phase method, a hydrothermal method, and a co-precipitation method have been proposed as methods for synthesizing LiMPO4. To increase the purity of LiMPO4 by removing the impurities included therein, normally, the post-synthesis lithium metal phosphate (LiMPO4) is washed (Patent Documents 3 and 4).
The Patent Documents 3 and 4 disclose that a cathode active material for a lithium secondary battery containing a lithium metal phosphate is washed with a pH buffer, in order to increase its purity by removing the impurities included therein. In particular, it is disclosed that when the lithium-metal phosphate is LiFePO4, it is possible to not elute Fe from LiFePO4 and reduce the impurity containing a divalent iron, such as FeSO4, FeO, Fe3(PO4)2. Further, it is also disclosed that it is possible to reduce impurities included in the LiFePO4, such as Li2CO3 or Li3PO4.
[Patent Document 1] JP Patent Publication No. 9-171827
[Patent Document 2] JP Patent Publication No. 9-134725
[Patent Document 3] JP Patent Publication No. 2009-32656
[Patent Document 4] JP Patent Publication No. 2009-259807